1. Field of the Invention
The present invention relates to an optical regenerator suitable for use with optical time division multiplexed (OTDM) signals carried on an optical network. The signals may be, for example, optical packets or a circuit switched data stream.
2. Related Art
In order to use fully the bandwidth available on optical communications networks, it is desirable to transmit time division multiplexed signals at a very high bit rate of tens or hundreds of Gbits per second. However, the very short duration pulses making up such signals soon suffer degradation in shape, timing and signal-to-noise ratio resulting, for example, from noise in optical amplifiers, dispersion in the optical transmission medium and/or from the effects of processing at nodes traversed by the packet. Therefore, if the extent of the optical network is not to be undesirably limited, it is necessary to use an optical regenerator to restore the timing and shape of the pulse train making up the optical signals. Ideally, the regenerator will function as a “3R” regenerator, that is it will re-amplify, re-time and re-shape the pulses. Examples of suitable optical regenerators are described in Lucek J and Smith K, Optics Letters, 18, 1226–28 (1993), and in Phillips I D, Ellis A D, Thiele H J, Manning R J and Kelly A E, Electronics Letters, 34, 2340–2342 (1998). The use of such techniques makes it possible to maintain the integrity of the optical data signals as they pass through a very large number of nodes. For example, Thiele H J, Ellis A D and Phillips I D, Electronics Letters, 35, 230–231 (1999) describe cascaded 40 Gbit/s 3R data regeneration in a recirculating loop. With a regenerator spacing of 100 km, the error-free transmission distance in the loop is extended by an order of magnitude, from 200 km to greater than 2000 km. Regenerators made from semiconductor non-linear optical devices, rather than fibre non-linear optical devices, are preferred because they are compact, stable, easily integrated, and operate at relatively low pulse energy.
Typically, an optical regenerator comprises an optical gate having a first optical input that receives an optical clock signal at the data line rate, and a second optical input, the control input that receives the data signal that is to be regenerated. Typically the gate, which includes a non-linear optical element, changes to a transmissive state when a binary digit ‘1’ occurs in the optical control signal that is applied, and reverts to the original non-transmissive state after a certain fixed time known as the gate window. The state of the gate is unchanged if a binary digit ‘0’ occurs in the optical control signal. The state of the non-linear element then determines whether a given-pulse in the optical clock train at the input to the gate is passed on to the output from the gate. In this way, the bit pattern in the input data stream is imposed on the optical clock train and output to form a regenerated optical data stream. However, while experiments reported in Kelly A E et al, Heatronics Letters, (in press, July 1999) have shown that semiconductor-based all-optical regenerators can function at bit rates as high as 80 Gbit/s it has been found that they are unable to perform satisfactorily at still higher bit rates, since then in general the bit period is very much less than the recovery time of the optical gate, so that the regenerated signals contain patterning effects which lead to bit errors.